invoices.rb

  def total_tax_amount_from_db
    total_amount = invoice_custom_vendor_items.where(is_taxable: true)
                                              .map(&:total_amount)
                                              .sum

    tax = Tax.new(zipcode: shipping_zip, revenue: total_amount).get_tax

    if tax['ItemMessages'].empty?
      taxes = tax['GroupList'][0]
      self.shipping_state  = taxes['StateCode']
      self.shipping_county = taxes['TaxList'][0]['CountyName']
      self.shipping_city   = taxes['TaxList'][0]['CityName']
      self.total_tax_rate  = taxes['TaxList'].map{ |t| t['TaxRate'] }.sum

      BigDecimal.new(tax['TotalTax'])
    else
      # Error message. Probably invalid ZIP code
      wrong_shipping_zip(tax['ItemMessages'][0]['Message'])
    end
  end
  
  def wrong_shipping_zip(error_message)
    errors.add(:shipping_zip, error_message)
    raise ActiveRecord::RecordInvalid.new(self)
  end